It's not that big a deal. Just means you have to wait for the opponent to commit or have a hard CC initiator go in before you use it. It's the same damage if they stay in it.

Are you ****ing serious lol? Not that big a deal?

look who doesn't play rumble often

I play Rumble like 80% of my games.

I save my ult until the enemy commits, and they're forced to stand in it if they want to kill me before I get a whole extra Q off. I save my ult until Jarvan or Amumu uses theirs or the enemy team has lined up in the jungle thinking they have the advantage. I save it for when I've hit 2 harpoons and the opponent still thinks he can trade with me or all-in me. I don't use it for the up-front burst unless I failed to get a kill without it, because I'm wasting most of its potential that way.---"That is why war is so tragic. To win means to make victims of your opponents and give birth to hatred." - Kratos Aurion, Tales of Symphonia